Fast Ethernet: 100Mbps Ethernet (IEEE 802.3u)

Fast Ethernet (100BASE-T) offers a speed increase ten times that of the 10BaseT Ethernet specification, while preserving such qualities as frame format, MAC mechanisms, and MTU. Such similarities allow the use of existing 10BaseT applications and network management tools on Fast Ethernet networks. Officially, the 100BASE-T standard is IEEE 802.3u.

Like Ethernet, 100BASE-T is based on the CSMA/CD LAN access method. There are several different cabling schemes that can be used with 100BASE-T, including:

• 100BASE-TX: two pairs of high-quality twisted-pair wires
• 100BASE-T4: four pairs of normal-quality twisted-pair wires
• 100BASE-FX: fiber optic cables

The Fast Ethernet specifications include mechanisms for Auto-Negotiation of the media speed. This makes it possible for vendors to provide dual-speed Ethernet interfaces that can be installed and run at either 10-Mbps or 100-Mbps automatically.

The IEEE identifiers include three pieces of information. The first item, "100", stands for the media speed of 100-Mbps. The "BASE" stands for "baseband," which is a type of signaling. Baseband signaling simply means that Ethernet signals are the only signals carried over the media system.

The third part of the identifier provides an indication of the segment type. The "T4" segment type is a twisted-pair segment that uses four pairs of telephone-grade twisted-pair wire. The "TX" segment type is a twisted-pair segment that uses two pairs of wires and is based on the data grade twisted-pair physical medium standard developed by ANSI. The "FX" segment type is a fiber optic link segment based on the fiber optic physical medium standard developed by ANSI and that uses two strands of fiber cable. The TX and FX medium standards are collectively known as 100BASE-X. 

The 100BASE-TX and 100BASE-FX media standards used in Fast Ethernet are both adopted from physical media standards first developed by ANSI, the American National Standards Institute. The ANSI physical media standards were originally developed for the Fiber Distributed Data Interface (FDDI) LAN standard (ANSI standard X3T9.5), and are widely used in FDDI LANs.

Protocol Structure - Fast Ethernet: 100Mbps Ethernet (IEEE 802.3u)The basic IEEE 802.3 Ethernet MAC Data Frame for 10/100Mbps Ethernet:

7	1	6	6	2	46 =< n =< 1500	4bytes
Pre	SFD	DA	SA	Length Type	Data unit + pad	FCS

• Preamble (PRE)- 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.
• Start-of-frame delimiter (SFD)- 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.
• Destination address (DA)- 6 bytes. The DA field identifies which station(s) should receive the frame..
• Source addresses (SA)- 6 bytes. The SA field identifies the sending station.
• Length/Type- 2 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format.
• Data- Is a sequence of n bytes (46=< n =<1500) of any value. (The total frame minimum is 64bytes.)
• Frame check sequence (FCS)- 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

Fast Ethernet standard is defined by IEEE (http://www.ieee.org ) in 802.3 & 802.3u.

http://standards.ieee.org/getieee802/download/802.3-2002.pdf : Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specification.
http://www.ethermanage.com/ethernet/descript-100quickref.html : Fast Ethernet Quick Guide